The present invention relates to an air-flow meter for an internal combustion engine and, more particularly, to an air-flow meter for controlling a fuel injector of an internal combustion engine with the air flow meter including and exposed hot wire arranged in an air passage of the internal combustion engine so as to measure the amount of suction air flow.
In order to maintain a proper combustion process in an internal combustion engine, the air-fuel ratio of a mixture to be fed into the internal combustion engine must be maintained at an appropriate value, and the amount of fuel to be injected into a combustion chamber by a fuel injector must be controlled in correspondence with the amount of air flow. Consequently, it is important to accurately sense the amount of air flow into the internal combustion engine.
In, for example, Japanese Laid Open Patent Application No. 56-18721, a hot-wire type air-flow meter for an internal combustion engine is proposed wherein, in order to prevent damage of a hot wire due to a backfire, a bypass air passage for bypassing a main air passage is provided, and the hot wire is arranged in the bypass air passage, with the amount of air flow being sensed on the basis of a change in the resistance of the hot wire. A disadvantage of this arrangement resides in the fact that dust particles flowing from the upper stream of the bypass air passage, pollutant particles contained in a reverse flow gas attributed to backward flow or backfire from the engine, etc. adhere to the hot wire, and an output signal from the hot wire changes with time, so as to result in a large error in the measured value of the amount of air flow.